Wall Street Journal: For more than a century, producing power has been a matter of flipping a switch. Need more electricity? Fire up some fuel. Need less? Dial the flame back down. Things won’t be that easy in a world that gets much of its energy from renewable sources, which come and go at nature’s whim. Wind tends to blow hardest at night — a problem, since people use electricity mostly during the day. Sunshine can lose its intensity in seconds if eclipsed by a cloud — inconvenient for people who like …